Casseyrain antenna system with rotatable main reflector for scanning

ABSTRACT

ANTENNA SYSTEM COMPRISING: A RADIATOR FIXED TO A BASE, ADAPTED FOR RADIATING AN ELECTROMAGNETIC WAVE AND HAVING A MAIN RADIATION AXIS, A SUB-REFLECTOR ALIGNED ON THE MAIN RODIATION AXIS, A MAIN REFLECTOR CAPABLE OF ROTATING ABOUT A REFLECTION AXIS OF THE ELECTROMAGNETIC WAVE FROM THE SUB-REFLECTOR AND ADAPTED FOR CONVERTING THE ELECTROMAGNETIC WAVE FROM THE SUB-REFLECTOR INTO A PLANE WAVE AND   EMITTING THE PLANE WAVE INTO THE AIR OR SPACE, AND A ROTATORY MECHANISM FOR ROTATING THE SUB-REFLECTOR AND THE MAIN REFLECTOR.

Feb. 9, 1971 Filed Feb. 19, 1969 MITSUO TANAKA EI'AL 3,562,753CASSEYRAIN ANTENNA SYSTEM WITH ROTATABLE MAIN v A REFLECTOR FOR SCANNINGTRANSMITTER-Y-RECHVER CHAMBH? INVENTOR 5 MITa/o TIA/HA! 4410 11 4;am/mull BY @4 am/ M ATTURNLYS FGb. 9,1971 rrsuo TANAKA ETAL 3,562,753

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TRANS/WER- u N Y ATTORNEYS United States Patent 3,562 753 CASSEYRAINANTENNA SYSTEM WITH ROTAT- ABLE MAIN REFLECTOR FOR SCANNING MitsuoTanaka, Kokubunji-shi, and Masao Kamimura, Kodaira-shi, Japan, assignorsto Hitachi, Ltd., Tokyo, Japan, a corporation of Japan Filed Feb. 19,1969, Ser. No. 800,636 Claims priority, application Japan, Feb. 23,1968, 43/11,125; June 17, 1968, 43/41,354 Int. Cl. H01g 1/28, 19/10 US.Cl. 343-705 6 Claims ABSTRACT OF THE DISCLOSURE The present inventionrelates to antenna systems and more particularly it pertains to antennasystems of the type having an antenna section and a transmitting andreceiving section which are arranged to rotate relatively with respectto each other.

In antenna systems such as an antenna having a large caliber for spacecommunications and being capable of scanning space wholly and a highgain transmitting and receiving antenna for a spin stabilized stationarysatellite, it is necessary for the transmission and reception of anelectromagnetic wave to rotate the antenna section of the system or toturn it in a particular direction.

Generally, in an antenna system capable of scanning space wholly, suchas a Cassegrain antenna, a horn reflector antenna, a Gregorian antennaor the like, it is impossible to fix a transmitter-receiver chamber tothe ground. This is because for the reception of a weak signal such asexperienced in space communications it is one of the essential factorsto make a wave-guide for interconnecting a radiator (used for eithertransmission or reception of electromagnetic waves) and atransmitter-receiver chamber as short as possible so that anyattenuation of an electromagnetic wave may be prevented. To this end, itis necessary to locate a transmitter-receiver chamber near to a radiatorand to form a wave guide as linear as possible for interconnecting theradiator and the transmitter-receiver chamber because otherwise thecharacteristics of the antenna system will be deteriorated. Furthermore,if the transmitter-receiver chamber is fixed to the ground so that onlythe antenna of large caliber may be movable, the interconnection of thechamber and the antenna is complicated with the mechanical strengththereof possibly decreased. Accordingly, in an antenna system of theabove-mentioned type, the antenna section and the transmitter-receiverchamber are constructed integrally with each other so that the radiatorand the transmitter-re ceiver means are in a fixed relation to eachother. As a result, the transmitter-receiver chamber can not but bearranged so as to be movable along with the antenna associatedtherewith. Thus, the antenna system of such a construction is not onlylarge-sized but also complicated to such an extent that steering,maintenance and inspection of the system are troublesome, and that toomuch complicated interconnection is needed for data-processinginformation to be transmitted or received by the system,

ice

e.g., several tens to several hundreds of interconnection wires areneeded for data processing.

Meanwhile, those antenna systems which have a main reflector turned tothe ground in the horizontal direction at a low elevation angle scanningnecessarily suffer from noise coming from the ground, which interfereswith normal reception of electromagnetic waves from an artificialsatellite. In those antenna systems which have a subreflector positionedin the center of the caliber of a main reflector, the field intensity ofthe main beam is lowered, the side lobe level in the vicinity of themain beam is contrariwise increased, which causes the system to be moresusceptible to noise from the ground and at the same time causes theantenna gain to be lowered.

As for antenna systems capable of being mounted on a spin stabilizedstationary satellite such as a despun antenna, since an antenna sectionmust be adapted so as to be always turned in a particular direction, theantenna section has to be rotated inversely with respect to thesatellite itself by a rotatory mechanism. For this purpose, a despunantenna has been conventionally provided with a rotary joint forinterconnecting a wave guide coupled to the output (or input) of atransmitter (or receiver) fixed to the body of the satellite and afeeder line leading to an antenna section (or receiving line from theantenna section). Therefore, from an electrical point of view, theantenna characteristics are deteriorated while, from a mechanical pointof view, the durability is questionable since the satellite itselfrotates at a speed near r.p.m. for a long time. Furthermore, in suchsystem, as a feeder line (or receiving line) is situated in the centerof the electromagnetic wave radiating face of the antenna section thefield intensity of the main beam is lowered, which is a large obstacleto far distance communications such as space communication.

One object of the present invention is to provide an antenna system ofthe type having an antenna section and a transmitting and receivingsection either one of which is movable relative to the other, yetdispensing with any rotary joint to be provided in a wave transmissionpath between the antenna section and the transmitting and receivingsection.

Another object of the present invention is to provide an improvedantenna system having an increased antenna. gain.

Still another object of the present invention is to provide an antennasystem in which the amount of an electromagnetic wave reflected to aradiator is extremely reduced so that the voltage standing wave ratio(VSWR) may be improved.

In order to achieve the abovementioned objects and other objects whichwill become apparent from the description on some embodiments of thepresent invention, a typical antenna system of the present inventioncomprises a radiator fixed to a base (the ground or an artificialsatellite) and adapted for radiating an electromognetic wave, asub-reflector shaped substantially in the form of a part of a quadricface and aligned on a main radiation axis of the radiator, a mainreflector shaped substantially in the form of a part of a quadric faceand adapted for converting an electromagnetic wave from thesub-reflector into a plane wave and emitting the converted wave into theair or space, a rotary mechanism for rotating a structure including thesub-reflector and the main reflector about the main radiation axis ofthe radiator, and another rotatory mechanism for rotating the mainreflector about a reflection axis of the sub-reflector.

For a complete understanding of the above-mentioned and other objects,features and advantages of the present invention, description will bemade of some preferred embodiments in conjunction with the accompanyingdrawings, in which:

FIG. 1A is a front view showing the outline of a structure in accordancewith an embodiment of the present invention;

FIG. 1B is a sectional view taken along line 1B1B in FIG. 1A;

FIG. 2 is a diagram showing various loci of scanning by the system shownin FIGS. 1A and 1B; and

FIGS. 3-7 are diagrams each illustrating the major part of a structurein accordance with an embodiment of the present invention.

Referring to FIGS. 1A and 1B, numeral 1 denotes a radiator (to be usedfor either transmission or reception of electromagnetic waves) fixedonto a base, for example, onto the ground and having a main radiationaxis 8 passing through the zenith, and 2 denotes a sub-reflector alignedon the main radiation axis 8 and shaped substantially in the form of apart of the concave face of an ellipsoid which has its focal points atpoints 3 and 4, the point 3 being concurrent with the phase center of anelectromagnetic wave radiated by the radiator 1. Numeral 5 denotes amain reflector for reflecting electromagnetic waves and shapedsubstantially in the form of a part of a paraboloid which has its focalpoint at point 4 and the axis of which is a vertical axis 10 passingthrough the point 4. Numeral 6 denotes supporting poles for thesubreflector, and 7 are horizontal rotation rails arranged so as to berotatable about the main radiation axis 8 at the time when the antennasystem operates to scan space wholly (i.e., at the time of the wholespace scanning operation of the antenna system). A rotatory mechanismassociated with the rails is not shown. Numeral 9 denotes an axis ofrotation of the main reflector 5, i.e., an axis of elevational rotationinclined by an angle of with respect to the main radiation axis 8. 11 isa rotary bearing of the main reflector 5, and 12 denotes atransmitter-receiver chamber fixed to the ground and interconnected withthe radiator 1 by a wave guide 13.

In the above-mentioned structure, as radiator 1 may be used an antennacapable of radiating a spherical wave such as a conical antenna,pyramidal antenna or linear antenna. The electromagnetic wave radiatedfrom such an antenna is not necessarily an exact spherical wave. Aspherical wave radiated from the radiator 1 and having its phase centerconcurrent with the point 3 which is one of the focal points of anellipsoid a part of which constitutes the sub-reflector 2 "as describedabove, is converted into a spherical Wave having its phase centerconcurrent with the point 4 which is the other one of the focal pointsof the ellipsoid by the sub-reflector 2 positioned on the line of themain radiation axis 8 and above the radiator 1. The spherical wavereflected by the sub-reflector 2 is then converted into a plane wave bythe main reflector shaped, as mentioned above, substantially in the formof a part of the concave portion of such a paraboloid that has its focalpoint concurrent with the point 4, which therefore corresponds to thephase center of the electromagnetic wave from the sub-reflector 2, andthe electromagnetic wave is emitted in the direction of the main axis ofthe paraboloid (the main reflector) 5. It is needless to say that uponemission of the plane wave from the main reflector the suporting poles6, the sub-reflector 2 and the radiator 1 are so arranged as not toobstruct the optical path for such emission of the plane wave. In thisembodiment, though the sub-reflector 2 and the main reflector 5 areexplained as being shaped (substantially) in the form of a part of aquadric face for the sake of simplicity in theoretical consideration ingeometrical optics, since practically, the reflectors are part of pathsfor an electromagnetic wave which has not exactly the samecharacteristics as those of light, it is necessary to correct the shapesof the quadric faces of the reflectors defined from the viewpoint ofgeometrical optics so that the reflectors may have the most suitablecurved faces for paths for an electromagnetic wave. Hereinafterthroughout the present specification, such corrected surface is calledquasi-quadric face.

Next, explanation will be given of various loci during the spacescanning operation of the antenna system described above referring toFIG. 2. The antenna system is rotatable about the main radiation axis(the rotation axis for horizontal scanning) 8, and at the same time, themain reflector is rotatable about the axis 9 of elevational rotation.Thus, the scanning area of the system can be varied depending upon theangle 0 formed between the main radiation axis 8 and the axis 9 ofelevational rotation. In FIG. 2, numerals 20 and 21 indicate a directionto the zenith and a horizontal direction respectively. Suppose thatelevational rotation is to be effected. If the angle 0 formed betweenthe axes 8 and 9 is 45 the scanning locus draws a circle 22 partly incontact with a circle 21 representative of a horizon and with the point20 representative of the zenith in the drawing. If angle is smaller than45, the scanning locus draws a circle 23, so that areas in the vicinityof the horizon can not be scanned. If the angle 0 is larger than 45 andsmaller than the scanning locus draws a curve 24, and if the 0 reaches90, the resulting scanning locus is a straight line as indicated by 25.It is therefore clear that by effecting rotation about the rotation axis8 for horizontal scanning with an angle of 0 fixed to a value between 45and 90, the whole space can be scanned by the antenna system. Sinceduring a low elevation angle scanning there is some possibility ofinterruption of the optical path for the plane wave by the sub-reflector2 and the supporting poles 6, it is desirable that the positioning ofthe main reflector 5 and the subreflector 2 and selection of the angle 0should be such that interruption of the optical path can be prevented.

Although the above-described embodiment is an application to an antennasystem having a large caliber and fixed to the ground for use in spacecommunications, the present invention can be also used as a high gaintransmitting and receiving antenna for a spin stabilized stationarysatellite, i.e., as a despun antenna by merely miniaturizing theconstruction described. In such a case, the antenna system is mounted onthe body of the satellite. Then, the radiator is fixed in a manner thatthe radiation axis of an electromagnetic wave is concurrent with theaxis of spin of the satellite while the main reflector is so arranged asto be rotatable inversely relative to the spin of the satellite by meansof a rotatory mechanism. By this arrangement the mounted antenna systemis operable so as to be always turned in a particular direction, so thatirrespective of the spin of the satellite, intercommunication betweenthe satellite and a particular station, for example, a particular areaon the earth is possible. Thus, the antenna system of the presentinvention can be used either as an antenna system for spacecommunication or as a despun antenna to be mounted on a artificialsatellite, and the construction and the operation are same in both uses.The only difference is whether the system is fixed to the ground ormounted on an artificial satellite. Accordingly, the followingdescription of embodiments refers to both cases, i.e., in one case thesystem is fixed on the ground and in the other case the system ismounted on a satellite. Only major parts of the system will be describedand illustrated as to the following embodiments.

In FIG. 3, the sub-reflector is constituted by a part of the concaveportion of a quasi-hyperboloid. In the figure, numeral 30 denotes aradiator fixed to a base (the ground or a satellite) and adapted forradiating a spherical Wave, 31 denotes a sub-rebector constituted by apart of a quasi-hyperboloid having its focal points at points 32 and 33,the point 32 being concurrent with the phase center of anelectromagnetic wave radiated from the radiator 30. Numeral 34 denotes amain reflector constituted by a part of a quasi-paraboloid which has itsfocal point concurrent with the point 33 and has a main axis passingthrough the point 33. Numerals 36 and 37 are an axis for horizontalrotation scanning and an axis for elecational rotation scanning,respectively.

A spherical wave having the phase center at the point 32 radiated fromthe raditor is converted into a spherical wave having the phase centerat the point 33 by the sub-reflector 31, and the latter spherical waveis converted into a plane Wave and emitted into the air or space by themain reflector 34.

In FIG. 4, the sub-reflector is constituted by a plane mirror. In thefigure, numeral denotes a radiator fixed to a base (the ground or asatellite) and adapted for radiating a sperical wave, 41 a sub reflectorconstituted by a plane mirror, 42 a phase center of a spherical waveradiated from the radiator 40, 43 a point which is in point-symmetryrelation to the point 42 with respect to the sub-reflector 41, 44 a mainreflector constituted by a part of a quasi-paraboloid which has itsfocal point at the point 43 and has a main axis concurrent with avertical axis 45 passing through the point 43, 46 an axis for horizontalrotation scanning, and 47 an axis for elevational rotation scanning.

It should be noted that through in FIGS. 3 and 4 the axis for horizontalrotation scanning is perpendicular to the axis tfOI elevational rotationscanning, these axes may be transverse to each other at a suitable angleother than at a right angle.

In FIG. 5, the sub-reflector is constituted by a part of the convexportion of a quasi-hyperboloid. In the figure, numeral 50 denotes aradiator fixed to a base (the ground or a satellite) and adapted forradiating a spherical wave, and 51 is sub-reflector constituted by apart of the convex portion of a quasi-hyperboloid which has its focalpoints at points '52 and 53. The sub-reflector 51 converts a sphericalwave radiated by the radiator 50 and having its phase center at point 52into a spherical Wave having its phase center at point 5 3, the latterspherical wave is then converted into a plane wave by a main reflector54 constituted by a part of a quasi-paraboloid which has its focal pointconcurrent with the point 53 and has a main axis passing through thepoint 53, and the plane wave is emitted into the air or space from themain reflector 54. Numerals 55 and 56 are an axis for horizontalrotation scanning and an axis for elevational rotation scanning,respectively.

In each of the foregoing embodiments, as radiator is used an antennawhich radiates a spherical wave, or an other antenna that radiates aplane wave such as a horn reflector antenna or a parabola antenna may beused. And, the electromagnetic wave to be radiated from such radiatorneed not be an exact plane wave.

Referring to FIG. 6 showing another embodiment, numeral 60 denotes atransmitter-receiver chamber fixed to a base (the ground or asatellite), and 61 denotes a radiator fixed to the base and adapted forradiating a plane wave such as for example, a horn reflector antenna,point 62 being a phase center of a radiated electromagnetic wave fromthe radiator 61. Numeral 63 denotes a sub-reflector constituted by apart of the concave portion of a qu-asi-paraboloid which has its focalpoint concurrent with the point 64 and has a main axis concurrent with avertical axis passing through the point 64, the sub-reflector 63 beingaligned on a main radiation axis 66 of the radiator '61 and above theradiator 61. Numeral 65 denotes a main reflector constituted by a partof a quasi-paraboloid Which has its focal point concurrent with thepoint 64 and has a main axis concurrent with a vertical axis passingthrough the point 64. Numerals 66 and 67 are an axis for horizontalrotation scanning and an axis for elevational rotation scanning,respectively. A plane Wave radiated by the radiator 61 having its phasecenter at the point 62 is converted into spherical wave having its phasecenter at the point 64 by the subreflector 63, and the spherical wave isthen converted into a plane wave by and emitted into the air or spacefrom the main reflector 65.

Referring to FIG. 7 in which the sub-refletcor is constituted by a partof the convex portion of a quasi-paraboloid, numeral 70 denotes asub-reflector constituted by a part of the convex portion of aquasi-paraboloid which has its focal point at a point 71. Thesub-reflector 70 converts a plane wave radiated from a radiator 71 intoa spherical Wave having its phase center concurent with the point 71.Numeral 72 denotes a main reflector constituted by a part of aquasi-paraboloid having its focal point concurrent with the point 71.The main reflector 72 converts the spherical wave reflected from thesubreflector into a plane Wave and emits the plane wave into the air orspace. Numerals 73 and 74 are an axis for horizontal rotation scanningand an axis for elevational rotation scanning, respectively.

In each of the embodiments the antenna system is explained as being atransmitting antenna, but it is clear that the system can also servesatisfactorily as a receiving antenna such as an antenna for receivingan electromagnetic wave from an artificial satellite in spacecommunications, and description thereon is here omitted since theoperation of the system is the same as in the foregoing embodiments.

As can be understood from the above description the antenna systemconstructed in accordance with the present invention provides thefollowing various functional effects and advantages:

when used as an antenna system having a large caliber for use in spacecommunicaitons:

(1) Since structurally, in the present invention, signal transmittingand receiving means and a radiator are fixed to the ground, many kindsof restrictions and complication in the system as have been experiencedin conventional systems in which transmitting means and a radiator arerotatable can be removed, thus facilitating the steering, maintenanceand inspection of the system;

(2) A radiator is always directed to the zenith, so that signaltransmission and reception at low elevation angles is scarcely subjectedto deterioration of the characteristics against noise;

(3) A sub-reflector and supporting poles therefor are arranged so as notto interrupt optical paths for an electromagnetic wave, so thatundesirable scattering of an electromagnetic wave due to the existenceof such reflector and supporting poles and any increase of side lobelevels do not take place, thus improving the antenna gain andcharacteristics against noise when scanning at low elevation angles;

(4) A sub-reflector is not situated in the optical path for anelectromagnetic wave, so that the dimensions of the sub-reflector can berelatively large to decrease power dissipated from a radiator throughthe vicinity of the sub-reflector thereby increasing the antenna gain;

(5) The amount of an electromagnetic wave reflected from a sub-reflectorto a radiator is extremely small, so that the characteristics defined bythe VSWR is excellent;

(6) A main reflector can be provided at a low position, so that it isless susceptible to the Wind power than the conventional system; and

(7) Supporting poles for a sub-reflector are so arranged as to bemounted on a rotary plate or rails horizontally provided on the ground,so that the supporting poles are always free from torque stemming fromthe weight of the sub-reflector in rotating operation, and thereforeimpression of undesirable stress which causes distortions in thesub-reflector as in the conventional Cassegrain antennas can be avoided.Accordingly, deterioration of characteristics due to deviation of themain beam which may be caused by deviation of the position of thesubreflector and by distortion at the surface of a main reflector issmall.

When used as a despun antenna:

(1) No movable joint is provided for interconnecting a satellite and anantenna system, so that generation of noise from and loss of gain due tosuch movable joint no longer exist. Mechanical breakage does not occur,either;

(2) There exists nothing that may be an obstacle to the main beam, sothat the electrical characteristics are improved, and thus the system ofthe present invention is very suitable for transmitting and receiving aweak electromagnetic wave; and

(3) There is no restriction to be put on the manner of rotation of themain reflector, so that control of the antenna section to turn it in anyparticular direction is facilitated.

The above-described embodiments are only for explanatory purpose andvarious other modifications are possible without departing from thespirit of the invention, so that the scope of the present invention isnot limited by such embodiments.

What is claimed is:

1. An antenna system having a radiator with a main radiation axisadapted for radiating an electromagnetic wave and reflector meanscharacterized in that said radiator is fixed to a base and saidreflector means consists of a sub-reflector having a reflection axis andaligned on said main radiation axis so as to reflect the electromagneticwave received from said radiator in the form of a spherical wave and amain reflector constituted by a quasiquadric face and adapted forconverting the spherical wave from said sub-reflector into a plane Waveso that the plane wave is emitted into the air or space from said mainreflector, and that the system is further provided with a first rotatorymechanism for rotating said main reflector about the reflection axis ofsaid sub-reflector and with a second rotatory mechanism for rotating astructure which includes said sub-reflector and said main reflectorabout said main radiation axis of said radiator.

2. An antenna system according to claim 1, characterized in that saidsub-reflector is constituted by a part of a quasi-quadric face.

3. An antenna system according to claim 1, characterized in that saidsub-reflector is constituted by a plane mirror.

4. An antenna system according to claim 1, characterized in that theangle between said main radiation axis of said radiator and saidreflection axis of said subreflector is selected so as to be within arange from to 5. An antenna system according to claim 1, characterizedin that said base is the ground and a transmitterreceiver chamberincluding a transmitter-receiver means connected with said radiator isfixed to the ground.

6. An antenna system according to claim 1, characterized in that saidbase is an artificial satellite and said main radiation axis is madeconcurrent with the axis of spin of said satellite.

References Cited UNITED STATES PATENTS 3,407,404 10/1968 Cook et a1.343-765 ELI LIEBERMAN, Primary Examiner M. NUSSBAUM, Assistant ExaminerUS. Cl. X.R.

